1. Field of the Invention
The present invention relates generally to varactor diodes and, more particularly, to a method of forming a varactor diode having a hyperabrupt junction profile.
2. Description of the Related Art
In a semiconductor, charge carriers (electrons or holes) diffuse from a high carrier-density region to a low carrier-density region. For this reason, charge carriers diffuse across the junction of an unbiased semiconductor diode to create a depletion region of ionized atoms, i.e., atoms which have lost their mobile carriers. Once this built-in potential Vbi has been established by the initial diffusion, it acts as a barrier to further diffusion.
If a reverse bias is imposed across the diode, the depletion region widens to expose a region of negative charges (due to acceptor atoms) on one side of the junction and a region of positive charges (due to donor atoms) on the other. The width of the depletion region is a function of the impurity doping levels of the diode junction. For example, if both sides of the junction are equally doped, the depletion region will extend an equal distance from the junction. With unequal doping levels, the depletion region will extend farther into the side which has the smaller impurity concentration.
The electric field is found by integrating the negative and positive charges. In contrast, the potential drop across the junction is found by a second charge integration or, equivalently, an integration of the electric field. If the doping concentration is constant, the electric field in the depletion region peaks at the junction and decreases linearly to the edges of the depletion region. In this case, the potential drop across the depletion region has a quadratic form.
If the reverse bias is increased to a breakdown voltage VBR, a large reverse current results because the electric field at the junction exceeds the dielectric strength of the diode""s semiconductor material. Covalent atomic bonds are ruptured, a large number of minority carriers are released and the diode is said to avalanche. The electric field and potential drop of depletion regions has been discussed by many authors (e.g., Singh, Jasprit., Semiconductor Devices, McGraw-Hill, Inc., New York, 1994, pp. 192-208). In contrast with diodes that are purposely intended to operate in breakdown, varactor diodes are generally configured to avoid breakdown over an operational reverse-bias range.
In a varactor diode, each side of the diode junction is conductive and the depletion region acts as a dielectric so that a reverse-biased semiconductor junction has the structure of a capacitor, i.e., two conducting regions separated by a dielectric. The capacitance depends directly on the junction area and inversely on the width of the depletion region, i.e., C=(∈A)/d in which c is the dielectric constant, A is the junction""s cross-sectional area and d is the width of the depletion region. The diode capacitance decreases with increased reverse bias because this change in bias causes the depletion width to increase. The capacitance ratio over a specified reverse bias range is generally referred to as the tuning ratio.
Varactors find utility in a variety of electronic circuits. For example, a varactor diode in a resonant circuit can control the frequency of a voltage-controlled oscillator (VCO) or the amplifier frequency in a receiver. Typically, VCOs and receiver amplifiers are tuned smoothly across their operating bands. Varactor diodes for these applications usually exhibit a junction capacitance that is proportional to an exponential power of the reverse-bias voltage Vr, e.g., C varies as (1/Vr)xc2xd. Abrupt-junction varactor diodes have uniform doping on each side of the junction with an abrupt transition at the junction. It will be appreciated from the aforementioned inverse square root relationship, as the applied voltage is increased by a factor of 4, the junction capacitance in an abrupt junction varactor diode will decrease by a factor of 2:1. Another way of expressing this relationship is that over the range of 1 to 4 volts, the capacitance-tuning ratio of an abrupt junction varactor diode is 2:1.
As more and more applications have been identified which require either faster tuning/frequency hopping speeds or low voltage operation, recent emphasis has been placed on the finding ways to increase the tuning ratios of varactor diodes so as to enable larger swings of capacitance or reactance with the same or reduced applied voltages. Such emphasis has culminated in the development of so-called hyperabrupt junction varactor diodes. In a conventional hyperabrupt-junction diode, the doping level increases as the junction is approached from either side, yielding an exponential relationship in which C varies as (1/(Vr+h)k, in which Vr is the biasing voltage, h is the height of the Schottky barrier, the parameter k defines the doping variation as a function of the distance with respect to the surface. Thus, as opposed to the abrupt junction case described above where the exponent k is exactly equal to one-half, a hyperabrupt profile is obtained when the exponent k is greater than xc2xd.
In FIG. 1A, there is shown one type of conventional hyperabrupt junction varactor diode generally indicated at 10 and comprising a cathode defined by an epitaxial layer 14 of semiconductor material (e.g., Si or GaAs) doped N type on an N+ substrate 12. A variably doped hyperabrupt region 16 is defined beneath an anode region 18, the latter being provided with an ohmic contact 20 of, for example, PtSi, to an overlying anode metalization layer 22.
While the conventional structure depicted in FIG. 1A is capable of producing hyperabrupt varactor diodes having capacitance-voltage tuning ratios of up to 12:1, these structures have demonstrated a high degree of variability on both a unit-to-unit and a lot-to-lot basisxe2x80x94even from the same vendor. Moreover, it has proved difficult to move past a tuning factor of 12:1. The capacitance-voltage response of a conventional hyperabrupt varactor structure, as exemplified by FIG. 1A, is depicted in FIG. 1B. For purposes of comparison, the capacitance voltage response of a conventional varactor is also shown in FIG. 1B.
The inventors herein are the first to appreciate that the aforementioned limitations in repeatability and tuning ratio are directly attributable to the processes which have heretofore been employed in the fabrication of hyperabrupt structures. Returning briefly to the conventional structure shown in FIG. 1A, it will be recalled that the variably doped hyperabrupt cathode region 16 directly underlies the anode region 18. In the illustrative structure of FIG. 1, the hyperabrupt region 16 is N-type while the anode is P-type. High energy ion implantation of phosphorous, either singly or multiply charged, is the generally accepted mode of creating hyperabrupt region 16 in the N-type cathode layer 14. By way of illustrative example, this ion implantation may be accomplished either through the already existing P+ anode region 18 or, alternatively, directly into the cathode layer 14 with a subsequent P+ implant or diffusion to form the anode structure. In either case, the implantation of hyperabrupt region 16 must generally be performed at very high energy levelsxe2x80x94either on the order of 150 keV to 350 keV in conjunction with multiply charged phosphorous ions (P++) to result in an effective singly charged level of 300 keV to 700 keV, or substantially higher implantation energy levels using a MeV implanter with single charged phosphorous ions. The implant(s), either the hyperabrupt cathode implant region 16 alone or in conjunction with the anode implant region 18, are then thermally activated via a furnace or a Rapid Thermal Annealing (RTA) heat cycle. The resulting hyperabrupt doping profile typically realized in the exemplary structure of FIG. 1A is depicted in FIG. 1C.
A principal disadvantage associated with both of the aforementioned techniques for defining the hyperabrupt region resides in the half-width of the Gaussian implant profile. Having a junction in depth, it has heretofore been impossible to satisfactorily form a high ratio hyperabrupt profile by implantation because the exponent k depends on the Gaussian distribution of the implantation used. Unfortunately, the higher the energy or effective energy, the larger the standard deviation of the ion scatter and the more graded the corresponding N-type hyperabrupt region.
An additional source of undesirable variation in the doping profile of the hyperabrupt region 16 is the high temperature heat cycle (typically 900-1100xc2x0 C.) associated with the electrical activation of the implanted cathode and anode regions 16 and 18. The high temperatures to which the structure is subjected causes the dopants to become substitutional rather than interstitial in the lattice, thereby promoting solid state diffusion and, ultimately, leading to a further flattening of and greater variability in the resultant junction profiles. Lastly, the simultaneous diffusion of the P+ anode 18 and the N-type hyperabrupt region 16 adversely affects the control of the doping concentration at their interface. It is difficult enough to control this crossing point of two exponential functions without adding the variation introduced by the unwanted diffusion of the ion implantation regions.
The aforementioned deficiencies are addressed, and an advance is made in the art, by a hyperabrupt junction varactor diode structure which comprises a cathode formed by a substrate of n-type semiconductor material having an epitaxial layer of n-type semiconductor material disposed thereon. The structure further includes an anode formed by deposition of a layer of strongly doped p-type semiconductor material onto a surface of the epitaxial layer, the anode being thereby joined to the cathode at a junction between them. The cathode further includes a non-uniformly doped n-type hyperabrupt region that extends from the junction into the epitaxial layer. The impurity concentration level of the hyperabrupt region increases in a direction toward the junction.
The non-uniformly doped hyperabrupt region is defined directly within the epitaxial layer of n-type semiconductor material. In accordance with a preferred embodiment of the present invention, this is achieved by a relatively low energy (i.e., on the order of 10-70 keV) singly charged ion implantation followed by a low temperature thermal activation step via, for example, rapid thermal annealing at a temperature that is preferably between from about 700xc2x0 to 800xc2x0 C.
In accordance with an especially preferred embodiment of the present invention, the p-type semiconductor material is grown epitaxially over the hyperabrupt region by UHVCVDxe2x80x94at a temperature that is preferably below 600xc2x0 C. Epitaxial growth at such low temperatures avoids, or at least minimizes, diffusion of the anode material or atoms thereof into or through the hyperabrupt implanted region.